The relatively low mass energy density is today the main limitation to the use of rechargeable lithium or sodium batteries in portable equipment such as portable electronic equipment or electric vehicles. This is largely linked to the performances of the materials which compose the batteries. Currently, the available materials for negative electrodes have a specific capacity comprised between 300 and 350 Ah/kg. The higher the specific capacity of a material, the more the mass energy density can be increased.
As lithium metal has a theoretical specific capacity greater than 3800 Ah/kg, its use as a material for negative electrodes appears promising.
Unfortunately, this material used as active material in a negative electrode has an undesirable tendency to grow in the form of dendrites during charging phases. In growing, these dendrites pass through the liquid electrolyte and can consequently cause a short-circuit by electrically connecting the positive electrode to the negative electrode.
In order to avoid the growth of the lithium metal in the form of dendrites during charging phases, lithium-ion batteries use a negative electrode material into which the lithium ion can be introduced during the charging phase and from which it can be released during the discharging phase. Graphite is an example of this. An illustration of the use of graphite as negative electrode material is given in document U.S. Pat. No. 5,053,297.
Nevertheless, graphite has the drawback of having a specific capacity which is much lower (theoretically 376 Ah/kg in the case of LiC6 for example) than that of lithium metal.
Alternatively, the lithium metal can be used in combination with a polymer electrolyte composed of a neutral matrix such as polyethylene oxide (also referred to as PEO) in which a lithium salt such as lithium bis(trifluoromethanesulphonyl)imide (also referred to as LiTFSI) is dissolved. This is the case with lithium-metal-polymer batteries (LMP batteries). In order to limit the risk of growth of the lithium metal in the form of dendrites, the thickness of the lithium metal is limited, generally between 30 and 100 μm, in the negative electrodes of LMP batteries. Thus, the lithium metal is laminated in the form of a ribbon on a polymer electrolyte film in order to obtain an anode compartment the useful charge density per unit of surface area of which is relatively low, typically between 1 and 10 mAh/cm2.
Moreover, in such a polymer electrolyte, the ionic conductivity of the electrolyte is obtained by the addition of LiTFSI salt. The mobility of this salt, in particular the anion, in the neutral matrix generates a salt concentration gradient during the passing of an ionic current, in fact, the transport number (i.e. the fraction of the current transported by the ion) for the cation is less than 1. This concentration gradient is higher the stronger the current density is. Now, the formation of lithium metal in the form of dendrites is promoted by steep concentration gradients.
A final solution for avoiding the growth of lithium metal in the form of dendrite which is described here is the use of a very hard solid electrolyte. The high hardness of these solid electrolytes has the advantage of preventing the dendrites formed from passing through the solid electrolyte and generating a short-circuit. In the article “Lithium metal stability in batteries with block copolymer electrolytes”, D. Hallinan et al. have calculated that a polymer electrolyte with a hardness of approximately 6 GPa is necessary in order to avoid the formation of dendrites (Journal of the Electrochemical Society, 160 (3) A464-A470 (2013).
Ceramic materials such as Lisicon (“Li super ionic conductor”) or Nasicon (“Na super ionic Conductor”), which can be used as solid electrolytes, have a hardness of approximately 6 GPa. Moreover, these ceramic materials do not depend on any salt dissolved in their matrix in order to have ion conduction properties as they are intrinsically conductors of ions: the ionic conduction is obtained only by cation transport through the crystalline structure of these ceramic materials and the conduction of electrons is negligible. As a result, their transport number is equal to 1, which discourages even more the growth of lithium metal in the form of dendrites.
The use of a ceramic electrolyte therefore opens the way for the use of a negative electrode with much greater thickness as the problem of the formation of dendrites does not arise. Thus, a negative electrode with a much higher surface area capacity can be obtained. The combination of a negative electrode with a very high surface area capacity with a positive electrode with very high capacity such as an air electrode (using oxygen in the air) or a sulphur electrode can therefore lead to a battery with a very high mass and volume energy density being obtained. In such a battery, metal, for example steel, is deposited on the ceramic electrolyte, for example by cathodic sputtering, in a thin layer in order to form a current collector. Later, the lithium metal is grown between the collector and the ceramic electrolyte.
In such a battery with a very high mass and volume energy density, the reduction of the cations during the charging phase occurs at the interface between the ceramic electrolyte and the active material that is the lithium metal. Thus, the growth of the lithium metal occurs from this interface in a single dimension.
Nevertheless, the lithium metal can grow in the form of a dense and uniform layer (see FIG. 7) or in the form of a stringy or porous deposition (see FIG. 6). The inventors have observed that a negative electrode made of lithium metal in a stringy or porous form has a resistance which increases rapidly with the number of charging and discharging cycles in comparison to a negative electrode made of lithium metal in a dense and uniform form.
They attribute this increase to a reduction in the active surface of the negative electrode which is determined by the interface between the lithium metal and the solid electrolyte. The reduction in this surface leads to a reduction in the capacity of the negative electrode and an increase in the resistance of the latter. When the layer of lithium created during the charging phase is dense and uniform, there is no increase in the resistance of the negative electrode and the latter can be used for a higher number of charging and discharging cycles.
Moreover, the inventors have observed that the growth of lithium in a stringy or porous form (not very dense form) reduced the capacity of the negative electrode by rendering a part of the lithium metal inaccessible during the discharging phase: this part of the lithium metal is not oxidized. For example, the inventors have analyzed a totally discharged negative electrode and have observed that 30% of the lithium metal formed during the charging phase was not accessible during the discharging phase and was present in the stringy or porous form. This is due to the fact that the lithium metal is no longer in effective contact with the solid electrolyte and can therefore not be oxidized to the Li+ cation during the discharging phase at the level of the interface. On this same negative electrode, all the parts which were present in the dense and uniform form with a good interface with the solid electrolyte were accessible during the discharging phase and could be totally consumed.
In researching a solution to this problem, the inventors have discovered that the use of an amorphous metal in order to form the current collector avoids the formation of not very dense lithium and only dense lithium is produced. The present invention is based on this discovery.